1. Field of the Invention
The present invention relates to a stabilized direct-current power supply device such as a series regulator or switching regulator. More particularly, the present invention relates to soft starting of a stabilized direct-current power supply device.
2. Description of the Prior Art
FIG. 5 shows an example of the configuration of a conventional stabilized direct-current power supply device. The stabilized direct-current power supply device of FIG. 5 is composed of an input terminal 1, an output terminal 2, a ground terminal 3, an error amplifier 4, a reference voltage circuit 5, a control terminal 6, a PNP-type transistor Q1, an NPN-type transistor Q2, and resistors R1 to R3. All these circuit elements are integrated into a semiconductor integrated circuit device.
First, the configuration of the stabilized direct-current power supply device of FIG. 5 will be described. The input terminal 1 is connected to the emitter of the transistor Q1. The collector of the transistor Q1 is connected to one end of the resistor R1 and to the output terminal 2. The other end of the resistor R1 is connected through the resistor R2 to the ground terminal 3. The node between the resistors R1 and R2 is connected to the inverting input terminal of the error amplifier 4, and the output end of the reference voltage circuit 5 is connected to the non-inverting input terminal of the error amplifier 4. The output terminal of the error amplifier 4 is connected to the base of the transistor Q2. The collector of the transistor Q2 is connected to the base of the transistor Q1, and the emitter of the transistor Q2 is connected through the resistor R3 to the ground terminal 3. The first supply power input terminal of the reference voltage circuit 5 is connected to the input terminal 1, and the second supply power input terminal of the reference voltage circuit 5 is connected to the ground terminal 3. The control terminal 6 is connected to the error amplifier 4. The output terminal 2 and the ground terminal 3 are connected to a capacitor Co and a load RL provided outside the semiconductor integrated circuit device.
Next, the operation of the stabilized direct-current power supply device of FIG. 5 will be described. The transistor Q1 converts an input voltage Vin fed in via the input terminal 1 into an output voltage Vo, and feeds the output voltage Vo to the output terminal 2. The resistors R1 and R2 constitute an output voltage detection circuit, which divides the output voltage Vo and delivers a divided voltage Vadj to the inverting input terminal of the error amplifier 4. On the other hand, the reference voltage circuit 5, operating on the input voltage Vin, produces a reference voltage Vref on the basis of the band-gap voltage, and feeds the reference voltage Vref to the no-inverting input terminal of the error amplifier 4.
The error amplifier 4 amplifies the difference between the reference voltage Vref and the divided voltage Vadj, and outputs the amplified difference to the base of the transistor Q2. The transistor Q2 amplifies the signal output from the error amplifier 4 and feeds it to the base of the transistor Q1 in order to control the collector-emitter voltage of the transistor Q1. Moreover, by controlling a voltage applied to the control terminal 6, the output of the error amplifier 4 is turned on and off.
As the amplified error signal output from the error amplifier 4 to the transistor Q2 increases, the collector current of the transistor Q2 increases. Accordingly, the base current of the transistor Q1 increases, and thus the collector-emitter voltage of the transistor Q1 decreases. Thus, the larger the amplified error signal, the higher the output voltage Vo.
The reference voltage Vref is so determined that, when the divided voltage Vadj is equal to the reference voltage Vref, the output voltage Vo is equal to the desired output voltage Vo*. Thus, feedback control is performed in such a way that the output voltage Vo is kept equal to the desired output voltage Vo*. The capacitor Co serves as an output phase compensating capacitor that stabilizes the output voltage Vo.
However, the stabilized direct-current power supply device configured as described above suffers from a problem ascribable to a large current that flows instantaneously at start-up. This problem will be described in detail below with reference to FIGS. 5 and 6A to 6D. FIGS. 6A to 6D are time charts of the voltages and current observed at relevant points in the stabilized direct-current power supply device of FIG. 5 when it is operated without a load (with the load RL removed).
When the input voltage Vin starts being supplied to the input terminal 1, as shown in FIG. 6A, the input voltage Vin rises. At the same time that the input voltage Vin rises, as shown in FIG. 6B, the reference voltage Vref rises. Here, it is assumed that the voltage Vc (not shown in FIGS. 6A to 6D) also rises in synchronism with the input voltage Vin. At this time, as the charge current Ico of the capacitor Co, a current close to the limit of the current capacity of the transistor Q1 flows instantaneously as shown in FIG. 6D. Thereafter, when the capacitor Co is completely charged, and the output voltage Vo completely rises as shown in FIG. 6C, the output current Io drops to almost zero. Accordingly, the output voltage Vo-to-output current Io characteristic of the stabilized direct-current power supply device of FIG. 5 is as shown in FIG. 7.
Since the stabilized direct-current power supply device of FIG. 5 has an output voltage Vo-to-output current Io characteristic as shown in FIG. 7, even if the current IL that flows through the load RL in actual operation is small, a peak current Iop flows instantaneously at start-up. For this reason, unless the supply power source that supplies electric power to the stabilized direct-current power supply device has a sufficiently high capacity to permit the peak current Iop to flow, the output voltage of the supply power source drops at the start-up of the stabilized direct-current power supply device, and may cause malfunctioning of other systems connected in parallel with the stabilized direct-current power supply device. This requires the supply power source to have a considerably higher capacity than needed in actual operation, and thus makes the supply power source expensive.
To avoid this, a stabilized direct-current power supply device has been proposed that is provided with a means for reducing the output current Io at start-up. FIG. 8 shows an example of the configuration of a conventional stabilized direct-current power supply device of this type. In FIG. 8, such circuit elements as are found also in FIG. 5 are identified with the same reference numerals and symbols, and their explanations will not be repeated.
The stabilized direct-current power supply device of FIG. 8 is obtained by providing the stabilized direct-current power supply device of FIG. 5 additionally with a time constant circuit 11. The time constant circuit 11 is composed of a resistor R8, a terminal 10, and a capacitor C2, and is provided between the reference voltage circuit 5 and the error amplifier 4. The resistor R8 and the terminal 10 are integrated into the semiconductor integrated circuit device, and the capacitor C2 is provided outside it. One end of the resistor R8 is connected to the output end of the reference voltage circuit 5, and the other end of the resistor R8 is connected to the non-inverting input terminal of the error amplifier 4 and to the terminal 10. The terminal 10 is grounded through the capacitor C2.
Next, the operation of the stabilized direct-current power supply device of FIG. 8 will be described with reference to FIGS. 8 and 9A to 9D. FIGS. 9A to 9D are time charts of the voltages and current observed at relevant points in the stabilized direct-current power supply device of FIG. 8 when it is operated without a load (with the load RL removed). For comparison, in FIGS. 9A to 9D, the voltages and current observed at the corresponding points in the stabilized direct-current power supply device of FIG. 5 are shown with broken lines.
When the input voltage Vin starts being supplied to the input terminal 1, as shown in FIG. 9A, the input voltage Vin rises. At the same time that the input voltage Vin rises, as shown in FIG. 9B, the reference voltage Vref rises, but more gently owing to the time constant circuit 11. Here, it is assumed that the voltage Vc (not shown in FIGS. 9A to 9D) also rises in synchronism with the input voltage Vin.
Since the reference voltage Vref rises gently, the capacitor Co is charged gently. Accordingly, as shown in FIG. 9C, the output voltage Vo rises gently, with a lower peak in the output current as shown in FIG. 9D. Thereafter, when the capacitor Co is completely charged, and the output voltage Vo completely rises, the output current Io drops to almost zero. Accordingly, the output voltage Vo-to-output current Io characteristic of the stabilized direct-current power supply device of FIG. 8 is as shown in FIG. 10. For comparison, in FIG. 10, the output voltage Vo-to-output current Io characteristic of the stabilized direct-current power supply device of FIG. 5 is shown with a broken line.
As described above, with the stabilized direct-current power supply device of FIG. 8, soft starting is possible when the input voltage Vin starts being supplied thereto. However, in a case where the operation of the stabilized direct-current power supply device is turned on and off by turning on and off the application of the voltage Vc to the control terminal 6 with the input voltage Vin and the reference voltage Vref both already risen, it is not possible to perform soft starting by exploiting the rise in the reference voltage Vref As a result, quite disadvantageously, when the application of the voltage Vc to the control terminal 6 is switched from off to on, the output current Io instantaneously becomes close to the peak current Iop.
An object of the present invention is to provide a stabilized direct-current power supply device that can be soft-started even when the reference voltage has already risen.
To achieve the above object, according to the present invention, a stabilized direct-current power supply device is provided with: a voltage conversion circuit for converting an input voltage into an output voltage to be fed out; a detection circuit for detecting the output voltage; a reference voltage circuit for generating a reference voltage; an error amplifier for comparing the detection voltage output from the detection circuit with the reference voltage and amplifying the difference therebetween; a control circuit for controlling the voltage conversion circuit according to the output signal of the error amplifier so as to keep the detection voltage equal to the reference voltage; and a current suppression circuit for suppressing the current output from the voltage conversion circuit when the output voltage rises.